A photoresistor, which is also called a light-dependent resistor (LDR) or photocell, consists of a piece of semiconductor material that exhibits an interesting characteristic: It acts like a resistor except that the value of resistance depends on how much light is shining on it.
An assortment of photoresistors
You can see an assortment of photoresistors in this figure, left, and a close-up of one photoresistor on the right. Note that these little devices have no identifying marks, such as a model number, which makes working with them a bit sketchy, er, loads of fun!
An assortment of photoresistorsPinning down the exact resistance of a photoresistor is like trying to catch a feather as it floats down through the air. Part of the problem is that you never really know exactly how much light is shining — that is, unless you happen to own a special device called a light meter, which measures light energy. (Add it to your holiday wish list or borrow one from a photographer friend!)
In general, a photoresistor works like this:
- In bright light, its resistance is relatively low (usually less than 10 kΩ).
- In darkness, its resistance is relatively high (usually more than 1 MΩ).
